As the resources of combustible fuels to supply energy for heating, cooling and electricity are becoming more depleted, considerable interest has been generated in the use of solar energy to satisfy these requirements. The ability of a solar powered system to effectively utilize solar energy in a reliable and economical manner depends in large part on (1) its ability to efficiently store large quantities of heat during the limited number of hours of available sunlight in order to operate the system during the time in which sunlight is not available; and (2) its ability to efficiently store heat at the highest possible temperature over a relatively wide range of collection temperatures during the available hours of sunlight.
Because water has proved to be one of the most economical storage mediums available from the present state of the art, most prior art solar energy storage systems use water as the storage medium. Since the amount of heat that can be stored in a fixed quantity of water is directly proportional to its temperature, it is desirable to have the water at the highest temperature possible in order to keep the quantity of water required for storage at a minimum. On the other hand, because a single hot water storage tank can absorb heat only when the temperature from the solar collector is higher than the temperature of the water in the storage tank and because the available temperature at the solar collector varies significantly over the normal hours of available sunlight, it is desirable to use multiple storage tanks which permit shifting the heat storage to another tank when one of the tanks will not absorb any more heat from the solar collector.
Multiple water storage tank systems have been proposed where the solar collector is connected to the appropriate storage tank through temperature controlled mechanical valves. These systems require that both the temperature of the heat output from the solar collector and the temperatures of the water in the tanks be sensed, and that an appropriate control system be provided so that the mechanical valves can be sequenced to transfer the solar collector heat output from one storage tank as its temperature approaches that of the output from the solar collector to another storage tank which will accept the heat output. This has necessarily required these systems to be complex and thus expensive to build and operate. Similar systems have been proposed which provide for the recovery of usable heat from these storage tanks, however, such heat recovery systems have suffered from the same drawbacks as the prior art storage systems.